Surface light source for emitting light from two surfaces and double-sided display device using the same

ABSTRACT

A surface light source device includes a light-emitting element which generates light, and a light guide plate which, causes light emitted from the light-emitting element to be incident from an end face, causes the light to be reflected between opposite plate surfaces to guide the light along the plate surfaces and make the light emerge from the plate surfaces, and transmits light incident from one plate surface toward the other plate surface. A first optical element is placed on one plate surface side of the light guide plate, transmits part of light emerging from one plate surface of the light guide plate toward a front side, and returns the remaining light to a rear side. A second optical element is placed on the other plate surface side and transmits at least part of light emerging from the other plate surface side.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority fromthe prior Japanese Patent Application No. 2002-175858, filed Jun. 17,2002, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a surface light source having flat exitsurfaces on its upper and lower surfaces and a double-sided displaydevice using the light source.

2. Description of the Related Art

As a surface light source having a flat exit surface, a light source isknown, which has a light-emitting element and a light guide plate whichallows light emitted from the light-emitting element to be incident froman end face, and causes the light to emerge from the front surface.

Conventionally, a surface light source of this type is comprised of alight guide plate which is formed from a transparent plate such as anacrylic resin plate and has one end face on which a light from a lightsource is incident, a front surface from which light transmitted throughthe transparent plate emerges, and a reflecting film formed on an entirerear surface facing the front surface of the transparent plate, and alight-emitting element serving as the light source formed from an LED(Light-Emitting Diode), cold cathode tube, or the like.

In this surface light source, light emitted from the light-emittingelement and incident on the light guide plate from the incident end faceis guided in the light guide plate while repeating internal reflectionof the light by the front surface of the light guide plate andreflection of the light by the rear surface of the light guide plate,and the light finally emerges from the front surface.

The above surface light source is used as an illumination light sourcefor a display device such as a liquid crystal display device. In adisplay device of this type, a display element which performs display bycontrolling transmission of light is placed on the front side of thesurface light source such that the observation surface of display facesthe opposite side to the surface light source.

Recently, as electronic equipment such as a cell phone or portableterminal device having a lid that can open and close with respect to anequipment body, an electronic equipment has appeared, which has displaysections on the inner surface (the surface that faces the equipment bodywhen the lid is closed) and the outer surface so as to be capable ofdisplaying information while the lid is open and closed.

A conventional surface light source is designed to make light emerge toonly the front side. In order to manufacture the electronic equipmenthaving display sections on the two surfaces, therefore, surface lightsources must be placed on the respective display sections.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a surface lightsource which can cause light to emerge to both the front side and therear side, and a display device which can perform display operationusing both one surface and the other surface as display surfaces byusing one surface light source.

In order to achieve the above object, according to the first aspect ofthe present invention, there is provided a surface light source devicecomprising a light-emitting element which generates light, a light guideplate which has a plate-like shape, causes light emitted from thelight-emitting element to be incident from one end face of the plate,causes the light to be reflected between plate surfaces of the plate toguide the light to the entire plate surfaces and make the light emergefrom the two plate surfaces, and transmits light incident from one platesurface toward the other plate surface, a first optical element which isplaced on one plate surface side of the light guide plate, transmitspart of light emerging from one plate surface of the light guide platetoward the front side, and returns the remaining light to a rear side,and a second optical element which is placed on the other plate surfaceside of the light guide plate and transmits at least part of lightemerging from the other plate surface side of the light guide plate.

This surface light source causes most of light emitted from thelight-emitting element and entering the light guide plate from its endface to emerge from the front surface of the light guide plate, andcauses part of the light to be transmitted through the first opticalelement placed on the front side of the light guide plate and emerge tothe front side. The surface light source also causes the first opticalelement to return the remaining light to the rear side, and causes thereturn light to be transmitted through the light guide plate and emergeto its rear surface. The surface light source then causes at least partof the light to be transmitted through the second optical element placedon the rear side of the light guide plate and emerge to the rear side.According to the surface light source, light can be made to emerge toboth the front side and the rear side.

In the surface light source of the present invention, it is preferablethat the first optical element placed on the front side of the lightguide plate be formed from a diffusion layer which diffuses incidentlight and causes the diffused light to emerge from both one surface andthe other surface, and the second optical element placed on the rearside of the light guide plate be a semitransparent reflective layer. Itis also preferable that the second optical element be formed from asemitransparent reflective layer to which a white pigment is added.

The surface light source of the present invention may have anarrangement in which the first optical element placed on the front sideof the light guide plate and the second optical element placed on therear side of the light guide plate are formed into polarizing/separatinglayers each of which reflects one of two different polarized lightcomponents of incident light and transmits the other polarized lightcomponent, and the polarizing/separating layers are arranged such thatlight reflected by one polarizing/ separating layer is transmittedthrough the other polarizing/separating layer. In this case, as thepolarizing/separating layer, a reflection polarizing plate is used,which transmits one of two linearly polarized light components havingorthogonal planes of polarization, and reflects the other linearlypolarized light component, or comprises a circularlypolarizing/separating element which separates light into a clockwisecircularly polarized light component and a counterclockwise circularlypolarized light component, and two λ/4 plates placed to sandwich thecircularly polarizing/separating layer.

In addition, the light guide plate preferably has, on the other platesurface, a diffusion surface which diffuses internally reflected light.

In this case, a refracting layer which refracts light emerging from thelight guide plate and transmitted through the first optical element in adirection substantially parallel to the normal to the front surface ofthe light guide plate is preferably placed outside the first opticalelement. More specifically, the refracting layer comprises a prism sheeton which oblong prism portions parallel to one end face of the lightguide plate at which the light-emitting element is placed are denselyarrayed parallel to each other.

The light guide plate may have an arrangement in which areflecting/refracting surface refracts light, internally reflected bythe plate surface, in a direction in which an angle with respect to thenormal to the plate surface of the light guide plate becomes smallerthan that when the light is regularly reflected by a surface parallel tothe front surface is formed on the other plate surface of the lightguide plate.

A display device according to the second aspect of the present inventioncomprises the surface light source device of the present inventiondescribed above, a first display element which is placed on one platesurface side of the surface light source device and performs displayoperation by controlling transmission of light, and a second displayelement which is placed on the other plate surface side of the surfacelight source device and performs display operation by controllingtransmission of light.

In the display device of the present invention, the first displayelement is placed on one side of the surface light source device of thepresent invention described above such that a surface on the oppositeside to the observation side of display opposes the surface light sourcedevice, and the second display element is placed on such that a surfaceopposite to the observation side of display opposes the surface lightsource device, thereby performing display operation using both onesurface and the other surface as display surfaces.

In this display device, it is preferable that the first and seconddisplay elements have different display areas, and the second displayelement have a smaller display area than the first display element.

In the display device of the present invention, at least one of thefirst and second display elements is preferably a semitransparentreflective display element which can perform both transmission displayrealized by controlling transmission of light emerging from the surfacelight source device and reflection display realized by controllingtransmission of external light incident from an observation side. Inthis case, the semitransparent reflective display element preferablyincludes a semitransparent reflective film formed from a porous metalfilm.

In this display device as well, it is preferable that the first opticalelement be formed from a diffusion layer which diffuses incident lightand causes the diffused light to emerge from both one surface and theother surface, and the second optical element be formed from asemitransparent reflective layer. Alternatively, it is preferable thatthe first and second optical elements be formed from polarizing/separating layers each of which reflects one of two different polarizedlight components of incident light and transmits the other polarizedlight component, and the polarizing/separating layers be arranged suchthat light reflected by one polarizing/separating layer is transmittedthrough the other polarizing/separating layer. As thispolarizing/separating layer, a reflection polarizing plate is preferablyused, which transmits one of two linearly polarized light componentshaving orthogonal planes of polarization, and reflects the otherlinearly polarized light component.

In addition, in this display device, a refracting layer which refractslight emerging from the light guide plate and transmitted through thefirst optical element in a direction substantially parallel to thenormal to the front surface of the light guide plate is preferablyplaced outside the first optical element. The refracting layer is formedfrom a prism sheet on which oblong prism portions parallel to one endface of the light guide plate at which the light-emitting element isplaced are densely arrayed parallel to each other, or realized byforming, on the other plate surface of the light guide plate, areflecting/ refracting surface which refracts light, internallyreflected by the plate surface, in a direction in which an angle withrespect to the normal to the plate surface of the light guide platebecomes smaller than that when the light is regularly reflected by asurface parallel to the front surface.

Additional objects and advantages of the invention will be set forth inthe description which follows, and in part will be obvious from thedescription, or may be learned by practice of the invention. The objectsand advantages of the invention may be realized and obtained by means ofthe instrumentalities and combinations particularly pointed outhereinafter.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

The accompanying drawings, which are incorporated in and constitute apart of the specification, illustrate embodiments of the invention, andtogether with the general description given above and the detaileddescription of the embodiments given below, serve to explain theprinciples of the invention.

FIG. 1 is a side view of a surface light source device according to thefirst embodiment of the present invention;

FIG. 2 is a side view of a display device according to the secondembodiment of the present invention;

FIG. 3 is an enlarged sectional view showing part of the first displayelement in the display device shown in FIG. 2;

FIG. 4 is an enlarged sectional view showing part of the second displayelement in the display device shown in FIG. 2;

FIGS. 5A and 5B are perspective views showing an example of electronicequipment having the above display device in different conditions;

FIG. 6 is a side view showing a surface light source according to thethird embodiment of the present invention; and

FIG. 7 is a side view showing a display device according to the fourthembodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Liquid crystal display devices will be described below as embodiments ofthe present invention with reference to the accompanying drawings.

[First Embodiment]

FIG. 1 shows a surface light source device according to the firstembodiment of the present invention. A surface light source device 10 ofthis embodiment includes a light-emitting element 11, light guide plate12, and first and second optical elements 14 and 15. The light guideplate 12 is placed to have one end face facing the light-emittingelement 11. The light guide plate 12 lets light emitted from thelight-emitting element 11 strike the end face and guides the light,while making it be internally reflected by one plate surface and theother plate surface, so as to make the light emerge from one platesurface. The light guide plate 12 also transmits light incident from oneplate surface and the other plate surface to the opposite sides. Thefirst optical element 14 is placed one plate side of the light guideplate 12. The first optical element 14 transmits part of the lightemerging from one plate surface of the light guide plate 12 and returnsthe remaining light to the other plate surface side. The second opticalelement 15 is placed on the other plate surface side of the light guideplate 12. The second optical element 15 transmits at least part of thelight emerging from the other plate surface of the light guide plate 12.

The light-emitting element 11 is a solid-state light-emitting elementincluding, for example, a red LED for emitting red light, a green LEDfor emitting green light, and a blue LED for emitting green light, whichare molded with a light scattering resin. By simultaneously turning onthe LEDs of the respective colors, white light obtained by mixing lightcomponents of the three colors, i.e., red, green, and blue, emitted fromthe LEDs emerges.

One or a plurality of light-emitting elements 11 formed from suchsolid-state light-emitting elements is or are placed to oppose one endface of the light guide plate 12.

In this embodiment, light emitted from the light-emitting element 11 isdirectly incident on the light guide plate 12. However, light emittedfrom the light-emitting element 11 may be diffused by a diffusion layerfirst, and then incident on the light guide plate. Alternatively, thelight-emitting element 11 may be placed to emit light in a differentdirection, and the emitted light is guided using a light guiding meanssuch as a mirror so as to make the light strike the light guide plate12. The light-emitting element 11 may be a cold cathode tube or thelike.

The light guide plate 12 is a transparent plate made of, for example, anacrylic resin, and has one end surface serving as an incident end face12 a on which light emitted from the light-emitting element 11 isincident. One plate surface 12 b (to be referred to as a front surface12 b hereinafter) of the light guide plate 12 is formed into a flatsurface. The other plate surface 12 c (to be referred to as a rearsurface 12 c hereinafter) of the light guide plate 12 is formed into adiffusion surface for diffusing internally reflected light on the rearsurface 12 c. In this embodiment, the rear surface 12 c of the lightguide plate 12 is a diffusion surface formed from an embossed surface onwhich fine projections 13 are formed at a very small pitch.

The first optical element 14 placed on the front side (front surface 12b side) of the light guide plate 12 is formed from a diffusion layer,e.g., a transparent resin film in which light scattering particles aredispersed, which diffuses light incident from one surface, and makes thediffused light emerge from the two surfaces, i.e., one surface and theother surface. The first optical element 14 will be referred to as adiffusion layer hereinafter.

The second optical element 15 placed on the rear side (rear surface 12 cside) of the light guide plate 12 is a white semitransparent reflectivelayer, e.g., a very thin, white film of a thickness of 0.05 to 0.075 mm,made of PET (polyethylene terephthalate) or the like containing whitepigment. The element 15 reflects and transmits light incident from onesurface at a reflectance and transmittance corresponding to the amountof white pigment added. The second optical element 15 will be referredto as a white semitransparent reflective layer hereinafter.

The diffusion layer 14 is placed on the front side of the light guideplate 12, with an air layer being interposed between the diffusion layer14 and the front surface 12 b of the light guide plate 12. The whitesemitransparent reflective layer 15 is placed on the rear side of thelight guide plate 12, with an air layer being interposed between thewhite semitransparent reflective layer 15 and the rear surface 12 c ofthe light guide plate 12.

The surface light source device 10 in this embodiment includes arefracting layer 16 which is placed on the front side (the opposite sideto the light guide plate of the diffusion layer 14) placed on the frontside of the light guide plate 12, and refracts light emerging to thefront side of the diffusion layer 14 in a direction (to be referred toas a direction near the normal hereinafter) substantially parallel tothe normal to the front surface 12 b of the light guide plate 12.

The refracting layer 16 is formed from a prism sheet obtained by formingnarrow, oblong prism portions 16 a on one surface of a transparent resinfilm made of an acrylic resin such that the prism portions are denselyarrayed parallel to each other on the entire region of the surface. Therefracting layer 16 will be referred to as a prism sheet hereinafter.

The prism sheet 16 is placed such that its surface on which the oblongprism portions 16 a are formed faces the front surface of the diffusionlayer 14. The ridges of the oblong prism portions 16 a are brought closeto or into contact with the front surface of the diffusion layer 14.

The surface light source device 10 causes light emitted from thelight-emitting element 11 and incident from the incident end face 12 aof the light guide plate 12 to emerge from the front surface 12 b of thelight guide plate 12. The device 10 causes part of the light to betransmitted through the diffusion layer 14 placed on the front side ofthe light guide plate 12 and emerge forward. The device 10 also reflectsand returns the remaining light to the rear side using the diffusionlayer 14, and causes the return light to be transmitted through thelight guide plate 12 and emerge from the rear surface 12 c. The device10 causes at least part of this light to be transmitted through thewhite semitransparent reflective layer 15 placed on the rear side of thelight guide plate 12 and emerge backward.

That is, light emitted from the light-emitting element 11 is incident onthe incident end face 12 a of the light guide plate 12, and guided inthe light guide plate 12 while internally reflected by the front surface12 b and rear surface 12 c of the light guide plate 12, as indicated bythe arrows in FIG. 1.

The internal reflection of light that has entered the light guide plate12 from the incident end face 12 a is caused by total reflection on theinterfaces between the front surface 12 b and rear surface 12 c of thelight guide plate and the air layer which is the open air; lightincident on the interface at an incident angle equal to or larger thanthe total reflection critical angle is reflected, and light incident onthe interface at an angle smaller than the total reflection criticalangle is transmitted through the interface and emerges.

In this embodiment, since the rear surface 12 c of the light guide plate12 is the diffusion layer formed from the above embossed surface, lightinternally reflected by the rear surface 12 c is diffused, and thediffused/reflected light is incident on the interface between the frontsurface 12 b of the light guide plate and the air layer.

As a consequence, most of the light that enters the light guide plate 12from the incident end face 12 a is incident on the interface between thefront surface 12 b and the air layer at an incident angle smaller thanthe total reflection critical angle and hence is transmitted through theinterface and emerges from the front surface 12 b of the light guideplate in the process of being guided into the light guide plate 12 whileinternally reflected by the front surface 12 b and rear surface 12 c ofthe light guide plate 12.

Of the light that enters the light guide plate 12 from the incident endface 12 a, light incident on the interface between the rear surface 12 cof the light guide plate 12 and the air layer at an incident anglesmaller than the total reflection critical angle is transmitted throughthe interface and emerges to the rear side of the light guide plate 12.

The light emerging from the front surface 12 b of the light guide plate12 is incident on the diffusion layer 14 placed on the front side of thelight guide plate 12 to be diffused. Of the light, diffused lightpropagating toward the front surface of the diffusion layer 14 istransmitted through the diffusion layer 14 and emerges from the frontsurface.

The diffused light emerging from the front surface of the diffusionlayer 14 is incident on the prism sheet 16 placed on the front side ofthe diffusion layer 14, and is refracted by the oblong prism portions 16a in a direction near the normal to the front surface 12 b of the lightguide plate 12. The light then emerges from the front surface of theprism sheet 16 to the front side.

The light emerging to the front side is light having a uniform luminancedistribution and directivity in the front-surface direction (a directionnear the normal to the front surface 12 b of the light guide plate 12),which has been diffused by the diffusion layer 14 and further refractedby the prism sheet 16 in a direction near the normal to the frontsurface 12 b of the light guide plate 12.

Of the light that has emerged from the front surface 12 b of the lightguide plate 12 and has been diffused by the diffusion layer 14, diffusedlight propagating toward the rear surface of the diffusion layer 14emerges from the rear surface of the diffusion layer 14 and returns tothe rear side.

This return light enters the light guide plate 12 from the front surface12 b, is transmitted through the light guide plate 12, and emerges fromthe rear surface 12 c to the rear side.

Of the return light that has been transmitted through the light guideplate 12 and has emerged to the rear side and the light that has enteredthe light guide plate 12 from the incident end face 12 a, light emergingto the rear side of the light guide plate 12 is incident on the whitesemitransparent reflective layer 15 placed on the rear side of the lightguide plate 12. Of this light, the amount of light corresponding to thereflectance of the white semitransparent reflective layer 15 isreflected by the white semitransparent reflective layer 15, and theamount of light corresponding to the transmittance of the whitesemitransparent reflective layer 15 is transmitted through the whitesemitransparent reflective layer 15 and emerges to the rear side.

The light emerging to the rear side is white light having a uniformluminance distribution, which has been diffused by the diffusion layer14 and transmitted through the white semitransparent reflective layer15.

The light reflected by the white semitransparent reflective layer 15again enters the light guide plate 12 from the rear surface 12 c and istransmitted through the light guide plate 12. The light then emergesfrom the front surface 12 b and strikes the diffusion layer 14 again.

As described above, the light that has struck the diffusion layer 14again is diffused by the diffusion layer 14 again. Of this light,diffused light propagating toward the front surface of the diffusionlayer 14 emerges from the front surface of the diffusion layer 14, isrefracted by the prism sheet 16 in a direction near the normal to thefront surface 12 b of the light guide plate 12, and emerges from thefront surface of the prism sheet 16 to the front side.

As described above, of the light that has been incident on the diffusionlayer 14 again and diffused again, return light emerging from the rearsurface of the diffusion layer 14 is transmitted through the light guideplate 12 and incident on the white semitransparent reflective layer 15.Of this light, the amount of light corresponding to the reflectance ofthe white semitransparent reflective layer 15 is reflected by the whitesemitransparent reflective layer 15, and the amount of lightcorresponding to the transmittance of the white semitransparentreflective layer 15 is transmitted through the white semitransparentreflective layer 15 and emerges to the rear side.

As described above, the surface light source device 10 makes thediffusion layer 14 diffuse light which is emitted from thelight-emitting element 11, enters the light guide plate 12 from theincident end face 12 a, and emerges from the front surface 12 b of thelight guide plate 12, and causes part of the diffused light to emerge tothe front side of the diffusion layer 14 while returning the remainingdiffused light to the rear side. The device 10 causes the return lightto be transmitted through the light guide plate 12 and incident on thewhite semitransparent reflective layer 15. The device 10 then causespart of the light to be transmitted through the white semitransparentreflective layer 15 and emerge to the rear side. According to thesurface light source device 10, light can be made to emerge to the twosides, i.e., the front side and the rear side.

The luminance ratio of light emerging to the front side to lightemerging to the rear side of the surface light source device 10 can bearbitrarily set by selecting degrees of diffusion in the forward andbackward directions of the diffusion layer 14 and a reflectance andtransmittance of the white semitransparent reflective layer 15.

In this embodiment, the diffusion layer 14 that receives incident lightand causes its diffused light to emerge from both one surface and theother surface is placed on the front side of the light guide plate 12 asthe first optical element which transmits part of light emerging fromthe front surface 12 b of the light guide plate 12 and returns theremaining light to the rear side, and the white semitransparentreflective layer 15 is placed on the rear side of the light guide plate12 as the second optical element which transmits at least part of lightemerging from the rear surface 12 c of the light guide plate 12. Thismakes it possible to make light having a uniform luminance distributionemerge to the front side and colorless, white light having a uniformluminance distribution emerge to the rear side.

In addition, in this embodiment, since the rear surface 12 c of thelight guide plate 12 is formed into a diffusion surface that diffuseslight internally reflected by the rear surface 12 c, light entering thelight guide plate 12 from the incident end face 12 a can be made toefficiently emerge from the front surface 12 b of the light guide plate12.

Furthermore, in this embodiment, since the prism sheet 16 serving as arefracting layer that refracts light emerging to the front side of thediffusion layer 14 in a direction near the normal to the front surface12 b of the light guide plate 12 is placed on the front side of thediffusion layer 14 placed on the front side of the light guide plate 12,light having directivity in the front-surface direction can be made toemerge to the front side.

[Second Embodiment]

FIG. 2 shows a display device using a surface light source deviceaccording to the present invention. FIGS. 3 and 4 are sectional viewsshowing portions of the first and second display elements used in thedisplay device. FIGS. 5A and 5B show an example of an electronicequipment having the display device.

In the second embodiment, a display device is formed by placing thefirst and second display elements on the two sides of the above surfacelight source device, and the display device is applied to electronicequipment such as a cell phone.

The electronic equipment shown in FIGS. 5A and 5B will be describedfirst. This electronic equipment is a cell phone, which includes anequipment body 1 having a plurality of keys 2 on its front surface, anda lid member 3 which has one end pivotally supported on the upper edgeportion of the equipment main body 1 so as to be openable/pivotal withrespect to the equipment main body 1. In this phone, a main displaysection 4 for displaying main information such as telephone numbers andelectronic mail is provided on the front surface of the lid member 3,i.e., the surface that is located in the same direction as that of thefront surface of the equipment main body 1 when the lid member 3 isopened as shown in FIG. 5A, and faces the equipment main body 1 when thelid member 3 is closed as shown in FIG. 5B. A sub-display section 5having a small screen which displays the time, the arrival of anincoming call, and the like is provided on the rear surface of the lidmember 3.

The display device shown in FIG. 2 will be described below. This displaydevice includes the surface light source device 10 described above, afirst display element 17 a which is placed on the front side of thesurface light source device 10 so as to make the surface on the oppositeside to the display observation side face the surface light sourcedevice 10 and displays an image upon controlling the transmission oflight, and a second display element 17 b which is placed on the rearside of the surface light source device 10 so as to make the surface onthe opposite side to the display observation side face the surface lightsource device 10 and displays an image upon controlling the transmissionof light.

The first and second display elements 17 a and 17 b are, for example,liquid crystal display devices. The first display element 17 a serves asa main display element. The second display element 17 b serves assub-display element.

According to this display device, in the lid member 3 of the electronicequipment (cell phone) shown in FIGS. 5A and 5B, the first displayelement or main display element 17 a serving as a main display elementis mounted to face the main display section 4 on the front surface ofthe lid member 3. The second display element or sub-display element 17 bserving as a sub-display element is mounted to face the sub-displaysection 5 on the rear surface of the lid member 3. The main displayelement 17 a is a liquid crystal display element having a screen sizecorresponding to the main display section 4 of the lid member 3. Thesub-display element 17 b is a liquid crystal display element having ascreen size corresponding to the small-screen sub-display section 5 ofthe lid member 3. The light guide plate 12, diffusion layer 14, prismsheet 16, and white semitransparent reflective layer 15 of the surfacelight source device 10 described above each have an area substantiallycorresponding to the entire region of the screen of the main displayelement 17 a. That is, these members have almost the same area in theplanar direction.

FIG. 3 is a sectional view of part of the main display element or unit17 a. FIG. 4 is a sectional view of part of the sub-display element orunit 17 b.

The main display element 17 a and sub-display element 17 b arereflection/transmission type display devices each having asemitransparent reflective film 21 on the opposite side to theobservation side of display. As shown in FIGS. 3 and 4, in each of thesedisplay devices, a liquid crystal layer 20 is placed between atransparent substrate 18 on the front side which is the observation sideof display and a transparent substrate 19 on the rear side which isopposite to the front-side substrate 18. The semitransparent reflectivefilm 21 is placed on the rear side of the liquid crystal layer 20.Polarizing plates 28 and 29 are arranged on the front side of the liquidcrystal layer 20 and the rear side of the semitransparent reflectivefilm 21, respectively.

The semitransparent reflective film 21 is made of, for example, a porousmetal film, and formed on the inner surface (opposite to the front-sidesubstrate 18) of the rear-side substrate 19.

The front-side substrate 18 is joined to the rear-side substrate 19through a frame-like seal member (not shown) at their peripheralportions. The liquid crystal layer 20 is placed in a region surroundedby the seal member between the front and rear substrates 18 and 19.

The main display element 17 a and sub-display element 17 b are STN(supertwisted nematic) type simple matrix liquid crystal displaydevices. In each device, a plurality of transparent scanning electrodes23 extending along the row direction (the horizontal direction in FIGS.3 and 4) are formed on the inner surface of one substrate, e.g., thefront-side substrate 18 (the surface opposite to the rear-side substrate19), so as to be spaced apart from each other in the column direction,and transparent signal electrodes 24 extending along the columndirection (the direction perpendicular to the drawing surface in FIGS. 3and 4) are formed on the inner surface of the rear-side substrate 19 soas to be spaced apart from each other in the row direction.

A transparent insulating film 22 is formed on the inner surface side ofthe rear-side substrate 19 so as to cover the semitransparent reflectivefilm 21. The signal electrodes 24 are formed on the insulating film 22.

Aligning films 25 and 26 are respectively formed on the innermostsurface sides of the front-side substrate 18 and rear-side substrate 19so as to cover the electrodes 23 and 24. The aligning direction of theliquid crystal molecules of the liquid crystal layer 20 which arelocated near the front and rear substrates 18 and 19 is defined by thealigning films 25 and 26, and the liquid crystal molecules aretwisted/aligned at a twist angle of 220 to 260° between the front andrear substrates 18 and 19.

The polarizing plates 28 and 29 are absorption polarizing plates eachdesigned to absorb one of two orthogonal linearly polarized lightcomponents of incident light and transmit the other polarized lightcomponent. The front-side polarizing plate 28 is bonded to the outersurface of the front-side substrate 18 with the transmission axis of theplate being oriented in a predetermined direction. The rear-sidepolarizing plate 29 is bonded to the outer surface of the rear-sidesubstrate 19 with the transmission axis of the plate being oriented in apredetermined direction.

The main display element 17 a is, for example, a color image displayunit having, on the inner surface of one substrate, e.g., the front-sidesubstrate 18, color filters 27R, 27G, and 27B of a plurality of colors,e.g., three colors of red, green, and blue, corresponding to a pluralityof pixel portions where the scanning electrodes 23 and signal electrodes24 oppose each other. These color filters are formed on the substratesurface of the front-side substrate 18. The scanning electrodes 23 areformed on the filters.

The sub-display element 17 b is, for example, a monochrome image displayunit having no color filter.

The main display element 17 a is placed on the front side of the surfacelight source device 10 such that the rear surface (the outer surface ofthe rear-side polarizing plate 29) of the main display element 17 aopposes a light exit region on the front surface 12 b of the light guideplate 12 of the surface light source device 10. The sub-display element17 b is placed on the rear side of the surface light source device 10such that the rear surface (the outer surface of the rear-sidepolarizing plate 29) of the sub-display element 17 b opposes a portionnear the center of a light exit region on the rear surface 12 c of thelight guide plate 12 of the surface light source device 10.

This display device is designed to perform display on the two displaysurfaces, i.e., one surface on which the main display element 17 a isplaced and the other surface on which the sub-display element 17 b isplaced. The main display element 17 a is driven when the surface onwhich it is placed serves as a display surface. The sub-display element17 b is driven when the surface on which it is placed serves as adisplay surface.

This display device is designed to perform transmission display usinglight from the surface light source device 10 under an environment whereexternal light with sufficient brightness cannot be obtained, and toperform reflection display using external light under an environmentwhere external light with sufficient brightness can be obtained. Thelight-emitting element 11 of the surface light source device 10 isturned off when reflection display is performed, and turned on whentransmission display is performed.

Display operation of this display device will be described byexemplifying display operation using the main display element 17 a. Whentransmission display using light from the surface light source device 10is to be performed, light which emerges on the front side of the surfacelight source device 10 and is incident on the main display element 17 afrom its rear side is converted into linearly polarized light along thetransmission axis of the rear-side polarizing plate 29 after a polarizedlight component along the absorption axis of the rear-side polarizingplate 29 of the main display element 17 a is absorbed by the rear-sidepolarizing plate 29. This light is then transmitted through thesemitransparent reflective film 21 and incident on the liquid crystallayer 20.

The polarizing stat of the linearly polarized light incident on theliquid crystal layer 20 is changed, in the process of being transmittedthrough the liquid crystal layer 20, by being subjected to abirefringence effect corresponding to the aligned state of the liquidcrystal molecules which is changed by an electric field applied betweenthe electrodes 23 and 24 at each pixel portion. In addition, lightcomponents of the light which have wavelengths falling in the absorptionwavelength bands of the color filters 27R, 27G, and 27B are absorbed bythem. The resultant colored light is incident on the front-sidepolarizing plate.28. If the linearly polarized light incident on thefront-side polarizing plate 28 is linearly polarized light along theabsorption axis of the front-side polarizing plate 28, the light isabsorbed by the polarizing plate 28. As a consequence, display on thecorresponding pixel portion becomes dark display (black). If this lightis linearly polarized light along the transmission axis of thefront-side polarizing plate 28, the light is transmitted through thepolarizing plate 28 and emerges to the front side. As a consequence,display on the corresponding pixel portion becomes bright display of acolor corresponding to one of the color filters 27R, 27G, and 27B whichis formed on the pixel portion.

When reflection display using external light is to be performed, lightincident on the main display element 17 a from the front side which isthe observation surface side of display is converted into linearlypolarized light along the transmission axis of the front-side polarizingplate 28 of the main display element 17 a after a polarized lightcomponent along the absorption axis of the front-side polarizing plate28 is absorbed by the front-side polarizing plate 28. The light is thentransmitted through the color filters 27R, 27G, and 27B. The resultantcolored light is incident on the liquid crystal layer 20.

The light incident on the liquid crystal layer 20 is subjected to abirefringence effect corresponding to the aligned state of liquidcrystal molecules at each pixel portion in the process of beingtransmitted through the liquid crystal layer 20. The resultant light isreflected by the semitransparent reflective film 21 formed on the innersurface of the rear-side substrate 19.

The light reflected by the semitransparent reflective film 21 istransmitted through the liquid crystal layer 20 and color filters 27R,27G, and 27B again to strike the front-side polarizing plate 28 again.If the linearly polarized light incident on the front-side polarizingplate 28 again is linearly polarized light along the absorption axis ofthe front-side polarizing plate 28, the light is absorbed by thepolarizing plate 28, and display of the corresponding pixel portionbecomes dark display (black). If the light is linearly polarized lightalong the transmission axis of the front-side polarizing plate 28, thelight is transmitted through the polarizing plate 28 to emerge to thefront side. As a consequence, display of the corresponding pixel portionbecomes bright colored display.

Display operation using the main display element 17 a has been describedabove. Transmission display and reflection display using the sub-displayelement 17 b are basically the same as those using the main displayelement 17 a. The sub-display element 17 b has no color filter, andhence bright display performed by this display element is colorless,white display.

That is, in this display device, the main display element 17 a andsub-display element 17 b are arranged on the front and rear sides of thesurface light source device 10 from which light emerges to the twosides, i.e., the front and rear sides, and light emerging to the frontside of the surface light source device 10 is incident on the maindisplay element 17 a to make the main display element 17 a performdisplay, while light emerging to the rear side of the surface lightsource device 10 is incident on the sub-display element 17 b to make thesub-display element 17 b perform display. This display device canperform display operation using, as display surfaces, both one surfaceon which the main display element 17 a is placed and the other surfaceon which the sub-display element 17 b is placed by using one surfacelight source device 10.

In this display device, the main display element 17 a is a color imagedisplay element having the color filters 27R, 27G, and 27B. As describedabove, however, the surface light source device 10 causes light having aluminance than that of light emerging to the rear side to emerge to thefront side, and hence a color image with sufficient brightness can bedisplayed on the main display element 17 a by causing the light having asufficient luminance to be incident on the main display element 17 a.

The sub-display element 17 b is a monochrome image display elementhaving no color filter, and hence no light is absorbed by color filters.Even if, therefore, the luminance of light emerging to the rear side ofthe surface light source device 10 is low, a monochrome image withsufficient brightness can be displayed on the sub-display element 17 b.

In addition, since the surface light source device 10 causes lightdiffused by the diffusion layer 14 and refracted by the prism sheet 16to emerge, the main display element 17 a can be made to perform displayoperation with sufficient front luminance without any luminanceirregularity.

Furthermore, since the surface light source device 10 causes light thatis diffused by the diffusion layer 14, returns to the rear side, and istransmitted through the white semitransparent reflective layer 15 toemerge to the rear side, the sub-display element 17 b can be made toperform colorless display without any luminance irregularity.

The display device of this embodiment uses reflection/transmission typedisplay devices as the main display element 17 a and sub-display element17 b, each of which has the semitransparent reflective film 21 on theopposite side to the observation side of display and performs bothtransmission display using light emerging from the surface light sourcedevice 10 and reflection display using external light incident from theobservation surface of display. Therefore, the main display element 17 aand sub-display element 17 b each can be made to perform transmissiondisplay using light from the surface light source device 10 andreflection display using external light.

In this embodiment, the main display element 17 a and sub-displayelement 17 b each have the semitransparent reflective film 21 betweenthe liquid crystal layer 20 and the rear-side polarizing plate 29, andreflection display using external light is performed by using only onefront-side polarizing plate 28. In this arrangement, therefore, theamount of light absorbed by the polarizing plate in reflection displaycan be decreased, and the brightness of reflection display can beincreased as compared with a case wherein the semitransparent reflectivefilm 21 is placed on the rear side of the rear-side polarizing plate 29.

In this embodiment, the main display element 17 a and sub-displayelement 17 b each have the semitransparent reflective film 21 formed onthe inner surface of the rear-side substrate 19. This arrangement candecrease the distance from the display surface (the front surface of thefront-side polarizing plate 28) of each of the display elements 17 a and17 b to the semitransparent reflective film 21, and hence prevents thegeneration of a double image, which is generated when an actuallydisplayed image shifts from a shadow of the displayed image which isprojected on the semitransparent reflective film 21, thereby displayinga high-quality image.

[Third Embodiment]

FIG. 6 shows a side surface of a surface light source device accordingto the third embodiment of the present invention.

As shown in FIG. 6, a surface light source device 30 of this embodimentincludes a light-emitting element 31, and a light guide plate 32. Theguide plate 32 causes light emitted from the light-emitting element 31to be incident from an end face of the light guide plate, causes most ofthe light to emerge from the front surface by guiding the light whileinternally reflecting it by the front and rear surfaces, and transmitslight incident from the front and rear surfaces. The device furtherincludes a first optical element 34 which is placed on the front side ofthe light guide plate 32, transmits part of the light emerging from thefront surface of the light guide plate 32, and returns the remaininglight to the rear side (light guide plate 32 side), and a second opticalelement 35 which is placed on the rear side of the light guide plate 32and transmits at least part of the light emerging from the rear surfaceof the light guide plate 32.

The light-emitting element 31 is identical to the light-emitting element11 of the surface light source device 10 of the first embodimentdescribed above, and hence a description thereof will be omitted.

The light guide plate 32 is a transparent plate made of, for example, anacrylic resin, and has one end surface serving as an incident end face32 a on which light emitted from the light-emitting element 31 isincident. One front surface 32 b of the light guide plate 32 is formedinto a flat surface. A rear surface 32 c is formed into areflecting/refracting surface which refracts light internally reflectedby the rear surface 32 c in a direction in which an angle with respectto the normal to the front surface 32 b of the light guide plate 32becomes smaller than that in a case wherein light is regularly reflectedby a surface parallel to the front surface 32 b (to be simply referredto as a direction in which an angle with respect to the normal decreaseshereinafter).

In this embodiment, the rear surface 32 c of the light guide plate 32 isformed into a reflecting/ refracting surface in the form of a prismsheet on which narrow, oblong prism portions 33 are densely arrayedparallel to each other throughout the entire region.

The first optical element 34 placed on the front side of the light guideplate 32 and the second optical element 35 placed on the rear side ofthe light guide plate 32 are reflecting/polarizing layers each designedto reflect one of two orthogonal polarized light components of incidentlight and transmit the other polarized light component. For example,each of these optical elements is a reflection polarizing plate having areflection axis and transmission axis in orthogonal directions. Thefirst optical element 34 on the front side of the light guide plate 32and the second optical element 35 on the rear side of the light guideplate 32 will be referred to as a front-side reflection polarizing plateand a rear-side reflection polarizing plate, respectively.

The front-side reflection polarizing plate 34 and rear-side reflectionpolarizing plate 35 are arranged such that directions (reflection axisdirections) parallel to the planes of polarization of reflected lightbecome substantially perpendicular to each other so as to make lightreflected by one reflection polarizing plate be transmitted through theother reflection polarizing plate.

The surface light source device 30 of this embodiment causes lightemitted from the light-emitting element 31 and incident from theincident end face 32 a of the light guide plate 32 to emerge from thefront surface 32 b of the light guide plate 32. The device 30 causespart of the light to be transmitted through the front-side reflectionpolarizing plate 34 placed on the front side of the light guide plate 32and emerge forward. The device 30 also reflects and returns theremaining light to the rear side using the front-side reflectionpolarizing plate 34, and causes the return light to be transmittedthrough the light guide plate 32 and emerge from the rear surface 32 c.The device 30 causes at least part of this light to be transmittedthrough the rear-side reflection polarizing plate 35 placed on the rearside of the light guide plate 32 and emerge backward.

That is, light emitted from the light-emitting element 31 is incident onthe incident end face 32 a of the light guide plate 32, and guided intothe light guide plate 32 while internally reflected by the front surface32 b and rear surface 32 c of the light guide plate 32, as indicated bythe arrows in FIG. 6.

The internal reflection of light that enters the light guide plate 32from the incident end face 32 a is caused by total reflection on theinterfaces between the front surface 32 b and rear surface 32 c of thelight guide plate and the air layer which is the open air. Lightincident on the interface at an incident angle equal to or larger thanthe total reflection critical angle is reflected, and light incident onthe interface at an angle smaller than the total reflection criticalangle is transmitted through the interface and emerges.

In this embodiment, since the rear surface 32 c of the light guide plate32 is the reflecting/refracting surface in the form of a prism sheet,light internally reflected by the rear surface 32 c is refracted in thedirection in which the angle with respect to the normal to the frontsurface 32 b of the light guide plate 32 decreases, and the reflectedlight is incident on the interface between the front surface 32 b of thelight guide plate and the air layer.

As a consequence, the light that enters the light guide plate 32 fromthe incident end face 32 a is incident on the interface between thefront surface 32 b and the air layer at an incident angle smaller thanthe total reflection critical angle and hence is transmitted through theinterface and emerges from the front surface 32 b of the light guideplate in the process of being guided into the light guide plate 32 whileinternally reflected by the front surface 32 b and rear surface 32 c ofthe light guide plate 32.

Of the light that enters the light guide plate 32 from the incident endface 32 a, light incident on the interface between the rear surface 32 cof the light guide plate 32 and the air layer at an incident anglesmaller than the total reflection critical angle is transmitted throughthe interface and emerges to the rear side of the light guide plate 32.

The light emerging from the front surface 32 b of the light guide plate32 is incident on the front-side reflection polarizing plate 34 placedon the front side of the light guide plate 32. Of the light, a polarizedlight component along the transmission axis of the front-side reflectionpolarizing plate 34 is transmitted through the front-side reflectionpolarizing plate 34 and emerges from the front surface to the frontside.

The light emerging to the front side is the light that has emerged fromthe front surface 32 b of the light guide plate 32, entered the lightguide plate 32 from the incident end face 32 a, and has been refractedin the direction in which the angle with respect to the normal to thelight guide plate front surface 32 b decreases by the light guide platerear surface 32 c formed into the reflecting/refracting surface in theform of a prim sheet so as to be internally reflected. This lighttherefore has directivity in the front-surface direction (a directionnear the normal to the front surface 32 b of the light guide plate 32).

Of the light that has emerged from the front surface 32 b of the lightguide plate 32 and struck the front-side reflection polarizing plate 34,a polarized light component along the reflection axis of the front-sidereflection polarizing plate 34 is reflected by the front-side reflectionpolarizing plate 34 to return to the rear side.

This return light is transmitted through the light guide plate 32 andemerges to the rear side. The light is further transmitted through therear-side reflection polarizing plate 35 placed on the rear side of thelight guide plate 32 and emerges to the rear side.

That is, since the front-side reflection polarizing plate 34 andrear-side reflection polarizing plate 35 are arranged such that therespective reflection axes are perpendicular to each other, the returnlight reflected by the reflection polarizing plate 34 is a linearlypolarized light component along the transmission axis of the rear-sidereflection polarizing plate 35. Most of the return light is thereforetransmitted through the rear-side reflection polarizing plate 35 andemerges to the rear side.

The light emerging to the rear side is part of the light that hasemerged from the front surface 32 b of the light guide plate 32 withdirectivity in the front-surface direction and has been reflected by thefront-side reflection polarizing plate 34, and hence is light havingdirectivity in the rear-surface direction opposite to the front-surfacedirection.

As described above, part of light entering the light guide plate 32 fromthe incident end face 32 a emerges to the rear side of the light guideplate 32. This light is, however, incident on the rear-side reflectionpolarizing plate 35. Of the light, a polarized light component along thetransmission axis of the rear-side reflection polarizing plate 35 istransmitted through the rear-side reflection polarizing plate 35 andemerges to the rear side, whereas a polarized light component along thereflection axis of the rear-side reflection polarizing plate 35 isreflected by the rear-side reflection polarizing plate 35 to betransmitted through the front-side reflection polarizing plate 34 andemerge to the front side.

As described above, the surface light source device 30 causes part oflight that is emitted from the light-emitting element 31, enters thelight guide plate 32 from the incident end face 32 a, and emerges fromthe front surface 32 b of the light guide plate 32 to be transmittedthrough the front-side reflection polarizing plate 34 and emerge to thefront side. In addition, the device 30 causes the front-side reflectionpolarizing plate 34 to reflect the remaining light and return it to therear side, and causes the return light to be transmitted through thelight guide plate 32. The device 30 further causes this light to betransmitted through the rear-side reflection polarizing plate 35 andemerge to the rear side. According to the surface light source device30, light can be made to emerge to both the front side and the rearside.

In the surface light source device 30 of this embodiment, as the firstand second optical elements respectively placed on the front and rearsides of the light guide plate 32, the reflection polarizing plates 34and 35 are used, each of which is designed to reflect one of twoorthogonal linearly polarized light components of incident light andtransmit the other polarized light component. This arrangement allowsmost of light that is emitted from the light-emitting element 31 andenters the light guide plate 32 from the incident end face 32 a to besubstantially equally distributed and emerge to the front and rear sideswithout any loss.

In addition, in this embodiment, since the rear surface 32 c of thelight guide plate 32 is formed into the reflecting/refracting surfacethat refracts light internally reflected by the rear surface 32 c in adirection in which an angle with respect to the normal to the frontsurface 32 b of the light guide plate 32 decreases, most of lightentering the light guide plate 32 from the incident end face 32 a can bemade to emerge from the front surface 32 b of the light guide plate 32in a direction near the normal to the front surface 32 b, therebycausing light having directivity in the front-surface direction or itsopposite direction to emerge to both the front side and the rear side.

[Fourth Embodiment]

The fourth embodiment of the present invention will be described next.FIG. 7 shows a side surface of a display device using the surface lightsource device according to the third embodiment. In this display device,a reflection/transmission type main display element (first displayelement) 17 a for displaying a color image, which is shown in FIG. 3, isplaced on the front side of the surface light source device 30 of thethird embodiment described above such that the surface on the oppositeside to the observation side of display is placed to face the surfacelight source device 30, and the transmission axis of a rear-sidepolarizing plate 29 becomes substantially parallel to the transmissionaxis of a front-side reflection polarizing plate 34 of the surface lightsource device 30. In addition, a reflection/transmission typesub-display element (second display element) 17 b for displaying amonochrome image, which is shown in FIG. 14, is placed on the rear sideof the surface light source device 30 such that the surface on theopposite side to the observation side of display faces the surface lightsource device 30, and the transmission axis of the rear-side polarizingplate 29 becomes substantially parallel to the transmission axis of arear-side reflection polarizing plate 35 of the surface light sourcedevice 30.

That is, in this display device, the first display element 17 a andsub-display element 17 b are respectively arranged on the front and rearsides of the surface light source device 30 which causes light to emergeto both the front side and the rear side. In this arrangement, lightemerging to the front side of the surface light source device 30 is madeincident on the first display element 17 a to make it perform displayoperation, and light emerging to the rear side of the surface lightsource device 30 is made incident on the sub-display element 17 b tomake it perform display operation. This display device can performdisplay operation by using, as display surfaces, both one surface onwhich the first display element 17 a is placed and the other surface onwhich the sub-display element 17 b is placed.

As described above, in the display device of this embodiment, thetransmission axis of the rear-side polarizing plate 29 of the firstdisplay element 17 a placed on the front side of the surface lightsource device 30 is made substantially parallel to the transmission axisof the front-side reflection polarizing plate 34 of the surface lightsource device 30, and the transmission axis of the rear-side polarizingplate 29 of the sub-display element 17 b is made substantially parallelto the transmission axis of the rear-side reflection polarizing plate 35of the surface light source device 30. For this reason, light emergingto the front side of the surface light source device 30, i.e., linearlypolarized light along the transmission axis of the front-side reflectionpolarizing plate 34, can be efficiently transmitted through therear-side polarizing plate 29 of the main display element 17 a and madeincident on a liquid crystal layer 20 of the main display element 17 a.In addition, light emerging to the rear side of the surface light sourcedevice 30, i.e., linearly polarized light along the transmission axis ofthe rear-side reflection polarizing plate 35, can be efficientlytransmitted through the rear-side polarizing plate 29 of the sub-displayelement 17 b and made incident on the liquid crystal layer 20 of thesub-display element 17 b.

In the display device of this embodiment, as described above, thesurface light source device 30 causes light emitted from alight-emitting element 31 to be substantially equally distributed andemerge to the front and rear sides, and also causes light to emerge withdirectivity in the front-surface direction or the rear-surface directionopposite to the front-surface direction. This allows both the maindisplay element 17 a and the sub-display element 17 b to perform displayoperation with sufficient brightness and sufficient front luminance.

In the display device of this embodiment, the main display element 17 aand sub-display element 17 b are reflection/transmission type displaydevices each having a semitransparent reflective film 21 on the oppositeside to the observation side of display. This arrangement allows each ofthe main display element 17 a and the sub-display element 17 b toperform both transmission display using light from the surface lightsource device 30 and reflection display using external light.

This embodiment uses reflection/transmission type display devices, eachhaving a semitransparent reflective film with the same arrangement asthat in the second embodiment, as the main display element 17 a andsub-display element 17 b. However, the present invention is not limitedto this. If the reflection polarizing plates 34 and 35 arranged on thefront and rear sides of a light guide plate 32 are used as reflectingplates for reflection display for the respective display elements 17 aand 17 b, the respective semitransparent reflective films can beomitted.

In the display device of this embodiment, the main display element 17 aand sub-display element 17 b each having the polarizing plates(absorption polarizing plates) 28 and 29 on its front and rear sides arearranged on the front and rear sides of the surface light source device30. However, since the surface light source device 30 causes linearlypolarized light along the transmission axis of the front-side reflectionpolarizing plate 34 to emerge to the front side and causes linearlypolarized light along the transmission axis of the rear-side reflectionpolarizing plate 35 to emerge to the rear side, even if the rear-sidepolarizing plates 29 are omitted from the main display element 17 a andsub-display element 17 b, the display elements 17 a and 17 b can be madeto perform display operation.

However, the main display element 17 a and sub-display element 17 bpreferably have the polarizing plates 28 and 29 on the front and rearsides as shown in FIGS. 3 and 4. In this arrangement, the rear-sidepolarizing plates 29 of the display elements 17 a and 17 b cause thelinearly polarized light emerging to the front and rear sides of thesurface light source device 30 to be incident, as linearly polarizedlight with a high degree of polarization, on the liquid crystal layer20, thereby increasing the contrasts of display on the display elements17 a and 17 b.

In the display device of this embodiment, the main display element 17 aplaced on the front side of the surface light source device 30 is acolor image display element having color filters 27R, 27G, and 27B, andthe sub-display element 17 b placed on the rear side of the surfacelight source device 30 is a monochrome image display element having nocolor filter. However, since the surface light source device 30 causeslight emitted from the light-emitting element 31 to be substantiallyequally distributed and emerge to the front and rear sides, both themain display element 17 a and sub-display element 17 b may be colorimage display elements having color filters.

In each of the display devices according to the second and fourthembodiments, each of the main display element 17 a and sub-displayelement 17 b is formed such that the semitransparent reflective film 21is formed on the inner surface of the rear-side substrate 19, and thetransparent electrodes 24 are formed on the semitransparent reflectivefilm 21 through the insulating film 22. However, the electrodes 24 maybe formed from porous metal films, and each electrode 24 may serve as asemitransparent reflective film.

The semitransparent reflective film 21 may be placed on the outersurface of the rear-side substrate 19 or the rear side of the rear-sidepolarizing plate 29 as long as the semitransparent reflective film 21 islocated on the opposite side to the observation side of display, i.e.,the rear side of the liquid crystal layer 20.

The display device of each embodiment described above usesreflection/transmission type display elements as the main displayelement 17 a and sub-display element 17 b. However, one or both of themain display element 17 a and sub-display element 17 b may betransmission type display elements which do not have the semitransparentreflective films 21 and are designed to perform only transmissiondisplay.

In the display device of each embodiment described above, both the maindisplay element 17 a and the sub-display element 17 b are STN typesimple matrix liquid crystal display elements. However, these displayelements 17 a and 17 b may be TN (twisted nematic) type, non-twisted,homogenously aligned, ferroelectric or antiferroelectric liquid crystaldisplay elements or the like or active matrix liquid crystal displayelements. Alternatively, they may be display elements other than liquidcrystal display elements.

In the surface light source device 10 of the first embodiment shown inFIG. 1, the rear surface 12 c of the light guide plate 12 is formed intothe diffusion layer for diffusing light internally reflected by the rearsurface 12 c, and the refracting layer (prism sheet) 16 for refractinglight emerging to the front side of the first optical element 14 in adirection near the normal to the front surface 12 b of the light guideplate 12 is placed on the front side of the first optical element(diffusion layer) 14 placed on the front side of the light guide plate12. However, the refracting layer 16 of the first optical element 14 maybe omitted by forming the rear surface 12 c of the light guide plate 12into a reflecting/refracting surface for refracting light internallyreflected by the rear surface 12 c in the direction in which the anglewith respect to the normal to the front surface 12 b of the light guideplate 12 decreases.

In the surface light source device 30 according to the third embodimentshown in FIG. 6, the rear surface 32 c of the light guide plate 32 isformed into a reflecting/refracting surface (in the form of a prismsheet) for refracting light internally reflected by the rear surface 32c in the direction in which the angle with respect to the normal to thefront surface 32 b of the light guide plate 32 decreases. However, therear surface 32 c of the light guide plate 32 may be formed into adiffusion surface for diffusing light internally reflected by the rearsurface 32 c, and a refracting layer for refracting light emerging tothe front side of the first optical element 34 in a direction near thenormal to the front surface 32 b of the light guide plate 32 may beplaced on the front side of the first optical element (front-sidereflection polarizing plate) 34 placed on the front side of the lightguide plate 32.

In the surface light source device 30 of the third embodiment, the firstand second optical elements 34 and 35 respectively arranged on the frontand rear sides of the light guide plate 32 are reflection polarizingplates each having reflection and transmission axes in orthogonaldirections. However, the first and second optical elements 34 and 35 maybe other kinds of polarizing/separating layers as long as they canreflect one of two different polarized light components of incidentlight and transmit the other polarized light component.

For example, polarizing/separating layers used as the first and secondoptical elements 34 and 35 may be polarizing/separating films (e.g.,cholesteric liquid crystal films) or the like which reflect one of twodifferent circularly polarized light components (a clockwise circularlypolarized light component and counterclockwise circularly polarizedlight component) of incident light and transmit the other circularlypolarized light component. In this case, the polarizing/separating filmused as the first optical element 34 and the polarizing/separating filmused as the second optical element 35 may be arranged such that lightreflected by one polarizing/separating film is transmitted through theother polarizing/separating film.

The above polarizing/separating layer may be formed by stackingretardation plates (λ/4 plates) to sandwich the polarizing/separatingfilm. In this case, linearly polarized light incident on thispolarizing/seperating layer is converted into circularly polarized lightto be incident on the polarizing/separating film, and the circularlypolarized light emerging from the polarizing/separating film isconverted into linearly polarized light.

In the surface light source device 10 of the first embodiment shown inFIG. 1, the second optical element 15 placed on the rear side of thelight guide plate 12 is a white semitransparent reflective layer. In thesurface light source device 30 of the third embodiment shown in FIG. 6,the second optical element 35 placed on the rear side of the light guideplate 32 is a reflecting/polarizing layer. However, the second opticalelements 15 and 35 may be semitransparent reflective layers formed fromporous metal films or the like.

Additional advantages and modifications will readily occur to thoseskilled in the art. Therefore, the invention in its broader aspects isnot limited to the specific details and representative embodiments shownand described herein. Accordingly, various modifications may be madewithout departing from the spirit or scope of the general inventiveconcept as defined by the appended claims and their equivalents.

1. A surface light source device comprising: a light-emitting elementwhich generates light; a light guide plate which has a plate-like shapeand comprises an end face and two opposing plate surfaces, wherein lightemitted from the light-emitting element is incident on the end face, andthe light guide plate causes the light to be reflected between the platesurfaces to guide the light along the plate surfaces, causes the lightto emerge from the two plate surfaces, and transmits light incident onone plate surface toward the other plate surface; a first opticalelement which is placed on a side of a first said plate surface side ofthe light guide plate, and which transmits a part of light emerging fromthe first plate surface of the light guide plate toward a front side,and returns the remaining light to a rear side; and a second opticalelement which is placed on a side of a second said plate surface of thelight guide plate, and which transmits at least a part of light emergingfrom the second plate surface of the light guide plate; and wherein thelight guide plate includes a diffusion surface at the second platesurface to diffuse internally reflected light.
 2. A device according toclaim 1, wherein the first optical element comprises a diffusion layerwhich diffuses incident light and causes the diffused light to emergefrom both surfaces thereof, and the second optical element comprises asemitransparent reflective layer.
 3. A device according to claim 2,wherein the second optical element comprises a semitransparentreflective layer having white pigment added thereto.
 4. A deviceaccording to claim 1, wherein the first and second optical elementscomprise polarizing/separating layers, each of which reflects one of twodifferent polarized light components of incident light and transmits theother polarized light component, and the polarizing/separating layersare arranged such that light reflected by one polarizing/separatinglayer is transmitted through the other polarizing/separating layer.
 5. Adevice according to claim 4, wherein the polarizing/separating layercomprises a reflection polarizing plate which transmits one of twolinearly polarized light components having orthogonal planes ofpolarization, and reflects the other linearly polarized light component.6. A device according to claim 4, wherein each polarizing/separatinglayer comprises: a circularly polarizing/separating element whichseparates light into a clockwise circularly polarized light componentand a counterclockwise circularly polarized light component; and two λ/4plates which sandwich the circularly polarizing/separating layer.
 7. Adevice according to claim 1, further comprising a refracting layer whichis provided outside the first optical element and refracts light, whichis transmitted through the first optical element and emerging from thelight guide plate, in a direction substantially parallel to a normal toa front surface of the light guide plate.
 8. A device according to claim7, wherein the refracting layer comprises a prism sheet including aplurality of oblong prism portions densely arrayed to be parallel toeach other and to said end face of the light guide plate.
 9. A deviceaccording to claim 1, wherein the light guide plate further comprises areflecting/refracting surface which is formed on the second platesurface, and which retracts light internally reflected by the secondplate surface in a direction in which an angle with respect to the anormal to the first plate surface of the light guide plate is smallerthan an angle with respect to the normal to the first plate surface whenlight is reflected by a surface parallel to the first surface.
 10. Asurface light source device comprising: a light-emitting element whichemits light; a light guide plate which has a platelike shape andcomprises an end face and two opposing plate surfaces, wherein lightemitted from the light-emitting element is incident on the end face, andthe light guide plate causes the light to be reflected between the platesurfaces to guide the light to the entire plate surfaces, and causes thelight to emerge from the two plate surfaces, and transmits lightincident on one plate surface toward the other plate surface; a firstoptical element which is placed on one a side of a first said platesurface side of the light guide plate, and which transmits a part oflight emerging from one the first plate surface of the light guide platetoward a front side, and returns the remaining light to a rear side; asecond optical element which is placed on a side of a second said platesurface of the light guide plate, and which transmits at least a part oflight emerging from the second plate surface of the light guide plate; afirst display element which is placed on a first plate surface side ofthe surface light source device, and which performs a display operationby controlling transmission of light; and a second display element whichis placed on a second plate surface side of the surface light sourcedevice, and which performs a display operation by controllingtransmission of light; wherein the light guide plate includes adiffusion surface at the second plate surface to diffuse internallyreflected light.
 11. A device according to claim 10, wherein the seconddisplay element has a smaller display area than the first displayelement.
 12. A device according to claim 10, wherein at least one of thefirst and second display elements comprises a semitransparent reflectivedisplay element which is adapted to perform both transmission display bycontrolling the transmission of light emerging from the surface lightsource device and reflection display realized by controllingtransmission of external light incident from an observation side.
 13. Adevice according to claim 12, wherein the semitransparent reflectivedisplay element comprises a semitransparent reflective film formed froma porous metal film.
 14. A device according to claim 10, wherein thefirst optical element comprises a diffusion layer which diffusesincident light and causes the diffused light to emerge from bothsurfaces thereof, and the second optical element comprises asemitransparent reflective layer.
 15. A device according to claim 10,wherein the first and second optical elements comprisepolarizing/separating layers, each of which reflects one of twodifferent polarized light components of incident light and transmits theother polarized light component, and the polarizing/separating layersare arranged such that light reflected by one polarizing/separatinglayer is transmitted through the other polarizing/separating layer. 16.A device according to claim 10, wherein each polarizing/separating layercomprises a reflection polarizing plate which transmits one of twolinearly polarized light components having orthogonal planes ofpolarization, and reflects the other linearly polarized light component.17. A device according to claim 10, further comprising a retractinglayer which is provided outside the first optical element and refractslight, which is transmitted through the first optical element andemerging from the light guide plate, in a direction substantiallyparallel to a normal to a front surface of the light guide plate.
 18. Adevice according to claim 10, wherein the refracting layer comprises aprism sheet including a plurality of oblong prism portions which areprovided on said end face of the light guide plate and which extend inparallel to each other at predetermined intervals.
 19. A deviceaccording to claim 10, wherein the light guide plate further comprises areflecting/retracting surface which is formed on the second platesurface, and which refracts light internally reflected by the secondplate surface in a direction in which an angle with respect to a normalto the first plate surface of the light guide plate is smaller than anangle with respect to the normal of the first plate surface when lightis reflected by a surface parallel to the first surface.
 20. A surfacelight source device comprising: a light-emitting element which generateslight; a light guide plate which has a plate like shape and comprises anend face and two opposing plate surfaces, wherein light emitted from thelight-emitting element is incident on the end face, and the light guideplate causes the light to be reflected between the plate surfaces toguide the light along the plate surfaces, causes the light to emergefrom the two plate surfaces, and transmits light incident on one platesurface toward the other plate surface; a first optical element which isplaced on a side of a first said plate surface of the light guide plate,and which transmits a part of light emerging from the first platesurface of the light guide plate toward a front side, and returns theremaining light to a rear side; and a second optical element which isplaced on a side of a second said plate surface of the light guideplate, and which transmits at least a part of light emerging from thesecond plate surface of the light guide plate; wherein the first andsecond optical elements comprise polarizing/separating layers, each ofwhich reflects, one of two different polarized light components ofincident light and transmits the other polarized light component, andthe polarizing/separating layers are arranged such that light reflectedby one polarizing/separating layer is transmitted through the otherpolarizing/separating layer.
 21. A surface light source devicecomprising: a light-emitting element which emits light; a light guideplate which has a platelike shape and comprises an end face and twoopposing plate surfaces, wherein light emitted from the light-emittingelement is incident on the end face, and the light guide plate causesthe light to be reflected between the plate surfaces to guide the lightto the entire plate surfaces, and causes the light to emerge from thetwo plate surfaces, and transmits light incident on one plate surfacetoward the other plate surface; a first optical element which is placedon a side of a first said plate surface of the light guide plate, andwhich transmits a part of light emerging from the first plate surface ofthe light guide plate toward a front side, and returns the remaininglight to a rear side; a second optical element which is placed on a sideof a second said plate surface of the light guide plate, and whichtransmits at least a part of light emerging from the second platesurface of the light guide plate; a first display element which isplaced on a first plate surface side of the surface light source device,and which performs a display operation by controlling transmission oflight; and a second display element which is placed on a second platesurface side of the surface light source device, and which performs adisplay operation by controlling transmission of light; wherein thefirst and second optical elements comprise polarizing/separating layers,each of which reflects one of two different polarized light componentsof incident light and transmits the other polarized light component, andthe polarizing/separating layers are arranged such that light reflectedby one polarizing/separating layer is transmitted through the otherpolarizing/separating layer.